1. Field of the Invention
Synthesis of many HIV protease inhibitors containing a hydroxyethylamine or hydroxyethylurea isostere include the amine opening of a key intermediate chiral epoxide. The synthesis of the key chiral epoxide requires a multi-step synthesis starting from L-phenylalanine and results in a low overall yield. The diastereoselectivity of the reduction step of the intermediate amino chloromethylketone is low and use of explosive diazomethane prevents the scale up of the method to multikilogram productions. The present invention relates to a method of preparing retroviral protease inhibitors and more particularly, to a diastereoselective method of forming chiral intermediates for the preparation of urea containing hydroxyethylamine protease inhibitors.
2. Related Art
Roberts et al, Science, 248, 358 (1990), Krohn et al, J. Med. Chem. 344, 3340 (1991) and Getman, et al, J. Med. Chem., 346, 288 (1993) have previously reported synthesis of protease inhibitors containing the hydroxyethylamine or hydroxyethylurea isostere which include the opening of an epoxide generated in a multi-step synthesis starting from an amino acid. These methods also contain steps which include diazomethane and the reduction of an amino chloromethyl ketone intermediate to an amino alcohol prior to formation of the epoxide. The overall yield of these syntheses are low and the use of explosive diazomethane additionally prevents such methods from being commercially acceptable.
Tinker et al U.S. Pat. No. 4,268,688 discloses a catalytic process for the asymmetric hydroformylation to prepare optically active aldehydes from unsaturated olefins. Similarly, Reetz et al U.S. Pat. No. 4,990,669 discloses the formation of optically active alpha amino aldehydes through the reduction of alpha amino carboxylic acids or their esters with lithium aluminum hydride followed by oxidation of the resulting protected beta amino alcohol by dimethyl sulfoxide/oxalyl chloride or chromium trioxide/pyridine. Alternatively, protected alpha amino carboxylic acids or esters thereof can be reduced with diisobutylaluminum hydride to form the protected amino aldehydes.
Reetz et al (Tet. Lett., 30, 5425 (1989) disclosed the use of sulfonium and arsonium ylides and their reactions of protected xcex1-amino aldehydes to form aminoalkyl epoxides. This method suffers from the use of highly toxic arsonium compounds or the use of combination of sodium hydride and dimethyl sulfoxide which is extremely hazardous in large scale. (Sodium hydride and DMSO are incompatible: Sax, N. I., xe2x80x9cDangerous Properties of Industrial Materialsxe2x80x9d, 6th Ed., Van Nostrand Reinhold Co., 1984, p. 433. Violent explosions have been reported on the reaction of sodium hydride and excess DMSO, xe2x80x9cHandbook of Reactive Chemical Hazardsxe2x80x9d, 3rd Ed., Butzerworths, 1985, p. 295. Matteson et al Synlett., 1991, 631 reported the addition of chloromethylithium or bromomethylithium to racemic aldehydes.
Tet. Letters, Vol. 27, No. 7, 1986, pages 795-798 discloses in general the oxidation of carbonyl compounds to epoxides or chlorohydrines using chloro- or bromomethyllithium. The reference however is silent about amino aldehydes as well as optically active compounds.
Human immunodeficiency virus (HIV), the causative agent of acquired immunodeficiency syndrome (AIDS), encodes three enzymes, including the well-characterized proteinase belonging to the aspartic proteinase family, the HIV protease. Inhibition of this enzyme is regarded as a promising approach for treating AIDS. One potential strategy for inhibitor design involves the introduction of hydroxyethylene transition-state analogs into inhibitors. Inhibitors adapting the hydroxyethylamine or hydroxyethylurea isostere are found to be highly potent inhibitors of HIV proteases. Despite the potential clinical importance of these compounds, previously there were no satisfactory synthesis which could be readily and safely scaled up to prepare large kilogram quantities of such inhibitors needed for development and clinical studies. This invention provides an efficient synthesis of intermediates which are readily amenable to the large scale preparation of hydroxyethylurea-based chiral HIV protease inhibitors.
Specifically, the method includes preparing a diastereoselective epoxide from a chiral alpha amino aldehyde.
This invention relates to a method of preparation of HIV protease inhibitor that allows the preparation of commercial quantities of intermediates of the formula 
wherein R1 is selected from alkyl, aryl, cycloalkyl, cycloalkylalkyl and arylalkyl, which are optionally substituted with a group selected from alkyl, halogen, NO2, OR9 or SR9, where R9 represents hydrogen or alkyl; and P1 and P2 independently are selected from amine protecting groups, including but not limited to, arylalkyl, substituted arylalkyl, cycloalkenylalkyl and substituted cycloalkenylalkyl, allyl, substituted allyl, acyl, alkoxycarbonyl, aralkoxycarbonyl and silyl. Examples of arylalkyl include, but are not limited to benzyl, ortho-methylbenzyl, trityl and benzhydryl, which can be optionally substituted with halogen, alkyl of C1-C8, alkoxy, hydroxy, nitro, alkylene, amino, alkylamino, acylamino and acyl, or their salts, such as phosphonium and ammonium salts. Examples of aryl groups include phenyl, naphthalenyl, indanyl, anthracenyl, durenyl, 9-(9-phenylfluorenyl) and phenanthrenyl, cycloalkenylalkyl or substituted cycloalkylenylalkyl radicals containing cycloalkyls of C6-C10. Suitable acyl groups include carbobenzoxy, t-butoxycarbonyl, iso-butoxycarbonyl, benzoyl, substituted benzoyl, butyryl, acetyl, tri-fluoroacetyl, tri-chloroacetyl, phthaloyl and the like.
Additionally, the P1 and/or P2 protecting groups can form a heterocyclic ring with the nitrogen to which they are attached, for example, 1,2-bis(methylene)benzene, phthalimidyl, succinimidyl, maleimidyl and the like and where these heterocyclic groups can further include adjoining aryl and cycloalkyl rings. In addition, the heterocyclic groups can be mono-, di- or tri-substituted, e.g., nitrophthalimidyl. The term silyl refers to a silicon atom optionally substituted by one or more alkyl, aryl and aralkyl groups.
Suitable silyl protecting groups include, but are not limited to, trimethylsilyl, triethylsilyl, tri-isopropylsilyl, tert-butyldimethylsilyl, dimethylphenylsilyl, 1,2-bis(dimethylsilyl)benzene, 1,2-bis(dimethylsilyl)ethane and diphenylmethylsilyl. Silylation of the amine functions to provide mono- or bis-disilylamine can provide derivatives of the aminoalcohol, amino acid, amino acid esters and amino acid amide. In the case of amino acids, amino acid esters and amino acid amides, reduction of the carbonyl function provides the required mono- or bis-silyl aminoalcohol. Silylation of the aminoalcohol can lead to the N,N,O-tri-silyl derivative. Removal of the silyl function from the silyl ether function is readily accomplished by treatment with, for example, a metal hydroxide or ammonium flouride reagent, either as a discrete reaction step or in situ during the preparation of the amino aldehyde reagent. Suitable silylating agents are, for example, trimethylsilyl chloride, tert-buty-dimethylsilyl chloride, phenyldimethylsilyl chloride, diphenylmethylsilyl chloride or their combination products with imidazole or DMF. Methods for silylation of amines and removal of silyl protecting groups are well known to those skilled in the art. Methods of preparation of these amine derivatives from corresponding amino acids, amino acid amides or amino acid esters are also well known to those skilled in the art of organic chemistry including amino acid/amino acid ester or aminoalcohol chemistry.
Preferably P1, P2 and R1 are independently selected from aralkyl and substituted aralkyl. More preferably, each of P1, P2 and R1 is benzyl.
Protected alpha-aminoaldehyde intermediates of the formula: 
and protected chiral alpha-amino alcohols of the formula: 
wherein P1, P2 and R1 are as defined above, are also described herein.
As utilized herein, the term xe2x80x9camino epoxidexe2x80x9d alone or in combination, means an amino-substituted alkyl epoxide wherein the amino group can be a primary, or secondary amino group containing substituents selected from hydrogen, and alkyl, aryl, aralkyl, alkenyl, alkoxycarbonyl, aralkoxycarbonyl, cycloalkenyl, silyl, cycloalkylalkenyl radicals and the like and the epoxide can be alpha to the amine. The term xe2x80x9camino aldehydexe2x80x9d alone or in combination, means an amino-substituted alkyl aldehyde wherein the amino group can be a primary, or secondary amino group containing substituents selected from hydrogen, and alkyl, aryl, aralkyl, alkenyl, aralkoxycarbonyl, alkoxycarbonyl, cycloalkenyl, silyl, cycloalkylalkenyl radicals and the like and the aldehyde can be alpha to the amine. The term xe2x80x9calkylxe2x80x9d, alone or in combination, means a straight-chain or branched-chain alkyl radical containing from 1 to about 10, preferably from 1 to about 8, carbon atoms. Examples of such radicals include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl, iso-amyl, hexyl, octyl and the like. The term xe2x80x9calkenylxe2x80x9d, alone or in combination, means a straight-chain or branched-chain hydrocarbon radial having one or more double bonds and containing from 2 to about 18 carbon atoms preferably from 2 to about 8 carbon atoms. Examples of suitable alkenyl radicals include ethenyl, propenyl, allyl, 1,4-butadienyl and the like. The term xe2x80x9calkoxyxe2x80x9d, alone or in combination, means an alkyl ether radical wherein the term alkyl is as defined above. Examples of suitable alkyl ether radicals include methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, iso-butoxy, sec-butoxy, tert-butoxy and the like. The term xe2x80x9ccycloalkenylxe2x80x9d, alone or in combination, means an alkyl radical which contains from about 3 to about 8 carbon atoms and is cyclic and which contains at least one double bond in the ring which is non-aromatic in character. The term xe2x80x9ccycloalkenylalkylxe2x80x9d means cycloalkenyl radical as defined above which is attached to an alkyl radical, the cyclic portion containing from 3 to about 8, preferably from 3 to about 6, carbon atoms. Examples of such cycloalkyl radicals include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and the like. Examples of such cycloalkenyl radicals include cyclopropenyl, cyclobutenyl, cyclopentenyl, cyclohexenyl, dihydrophenyl and the like. The term xe2x80x9carylxe2x80x9d, alone or in combination, means a carbocyclic aromatic system containing one, two or three rings wherein such rings may be attached together in a pendent manner or may be fused. Examples of xe2x80x9carylxe2x80x9d include phenyl or naphthyl radical either of which optionally carries one or more substituents selected from alkyl, alkoxy, halogen, hydroxy, amino, nitro and the like, as well as p-tolyl, 4-methoxyphenyl, 4-(tert-butoxy)phenyl, 4-fluorophenyl, 4-chlorophenyl, 4-hydroxyphenyl, 1-naphthyl, 2-naphthyl, and the like. The term xe2x80x9caralkylxe2x80x9d, alone or in combination, means an alkyl radical as defined above in which one hydrogen atom is replaced by an aryl radical as defined above, such as benzyl, 2-phenylethyl and the like. Examples of substituted aralkyl include 3,5-dimethoxybenzyl bromide, 3,4-dimethoxybenzyl bromide, 2,4-dimethoxybenzyl bromide, 3,4,5-trimethoxybenzyl bromide, 4-nitrobenzyl iodide, 2,6-dichlorobenzyl bromide, 1,4-bis(chloromethyl)benzene, 1,2-bis(bromomethyl)benzene, 1,3-bis(chloromethyl)-benzene, 4-chlorobenzyl chloride, 3-chlorobenzyl chloride, 1,2-bis(chloromethyl)benzene, 6-chloropiperonyl chloride, 2-chlorobenzyl chloride, 4-chloro-2-nitrobenzyl chloride, 2-chloro-6-fluorobenzyl chloride, 1,2-bis(chloromethyl)-4,5-dimethylbenzene, 3,6-bis(chloromethyl)durene, 9,10-bis(chloromethyl)anthracene, 2,5-bis(chloromethyl)-p-xylene, 2,5-bis(chloromethyl)-1,4-dimethoxybenzene, 2,4-bis(chloromethyl)anisole, 4,6-(dichloromethyl)-m-xylene, 2,4-bis(chloromethyl)mesitylene, 4-(bromomethyl)-3,5-dichlorobenzophenone, n-(alpha-chloro-o-tolyl)-benzylamine hydrochloride, 3-(chloromethyl)benzoyl chloride, 2-chloro-4-chloromethyltoluene, 3,4-dichlorobenzyl bromide, 6-chloro-8-chloromethylbenzo-1,3-dioxan, 4-(2,6-dichlorobenzylsulphonyl)benzylbromide, 5-(4-chloromethylphenyl)-3-(4-chlorophenyl)-1,2,4-oxadiazole, 5-(3-chloromethylphenyl)-3-(4-chlorophenyl)-1,2,4-oxadiazole, 4-(chloromethyl)benzoyl chloride, di(chloromethyl)toluene, 4-chloro-3-nitrobenzyl chloride, 1-(dimethylchlorosilyl)-2-(p,m-chloromethylphenyl)ethane, 1-(dimethylchlorosilyl)-2-(p,m-chloromethylphenyl)ethane, 3-chloro-4-methoxybenzyl chloride, 2,6-bis(chloromethyl)-4-methylphenol, 2,6-bis(chloromethyl)-p-tolyl acetate, 4-bromobenzyl bromide, p-bromobenzoyl bromide, alpha alphaxe2x80x2-dibromo-m-xylene, 3-bromobenzyl bromide, 2-bromobenzyl bromide, 1,8-bis(bromomethyl)naphthalene, o-xylylene dibromide, p-xylylene dibromide, 2,2xe2x80x2-bis(bromomethyl)-1,1xe2x80x2-biphenyl, alpha,alphaxe2x80x2-dibromo-2,5-dimethoxy-p-xylene, benzyl chloride, benzyl bromide, 4,5-bis(bromomethyl)phenanthrene, 3-(bromomethyl)benzyltriphenylphosphonium bromide, 4-(bromomethyl)benzyltriphenylphosphonium bromide, 2-(bromomethyl)benzyltriphenylphosphonium bromide, 1-(2-bromoethyl)-2-(bromomethyl)-4-nitrobenzene, 2-bromo-5-fluorobenzylbromide, 2,6-bis(bromomethyl) fluorobenzene, o-bromomethylbenzoyl bromide, p-bromomethyl benzoyl bromide, 1-bromo-2-(bromomethyl)naphthalene, 2-bromo-5-methoxybenzyl bromide, 2,4-dichlorobenzyl chloride, 3,4-dichlorobenzyl chloride, 2,6-dichlorobenzyl chloride, 2,3-dichlorobenzyl chloride, 2,5-dichlorobenzyl chloride, methyldichlorosilyl(chloromethylphenyl)ethane, methyldichlorosilyl(chloromethylphenyl)ethane, methyldichlorosilyl(chloromethylphenyl)ethane, 3,5-dichlorobenzyl chloride, 3,5-dibromo-2-hydroxybenzyl bromide, 3,5-dibromobenzyl bromide, p-(chloromethyl)phenyltrichlorosilane, 1-trichlorosilyl-2-(p,m-chloromethylphenyl)ethane, 1-trichlorosilyl-2-(p,m-chloromethylphenyl)ethane, 1,2,4,5-tetrakis(bromomethyl)benzene. The term aralkoxycarbonyl means an aralkoxyl group attached to a carbonyl. Carbobenzoxy is an example of aralkoxycarbonyl. The term xe2x80x9cheterocyclic ring systemxe2x80x9d means a saturated or partially unsaturated monocyclic, bicyclic or tricyclic heterocycle which contains one or more hetero atoms as ring atoms, selected from nitrogen, oxygen, silicon and sulphur, which is optionally substituted on one or more carbon atoms by halogen, alkyl, alkoxy, oxo, and the like, and/or on a secondary nitrogen atom (i.e., xe2x80x94NHxe2x80x94) by alkyl, aralkoxycarbonyl, alkanoyl, phenyl or phenylalkyl or on a tertiary nitrogen atom (i.e. =Nxe2x80x94) by oxido and which is attached via a carbon atom. Examples of such heterocyclic groups are pyrrolidinyl, piperidinyl, piperazinyl, morpholinyl, thiamorpholinyl, pyrrolyl, phthalimide, succinimide, maleimide, and the like. Also included are heterocycles containing two silicon atoms simultaneously attached to the nitrogen and joined by carbon atoms. The term xe2x80x9calkylaminoxe2x80x9d alone or in combination, means an amino-substituted alkyl group wherein the amino group can be a primary, or secondary amino group containing substituents selected from hydrogen, and alkyl, aryl, aralkyl, cycloalkyl, cycloalkylalkyl radicals and the like. The term xe2x80x9chalogenxe2x80x9d means fluorine, chlorine, bromine or iodine. The term dihaloalkyl means two halogen atoms, the same or different, substituted on the same carbon atom. The term oxidizing agent includes a single agent or a mixture of oxidizing reagents. Examples of mixtures of oxidizing reagents include sulfur trioxide-pyridine/dimethylsulfoxide, oxalyl chloride/dimethyl sulfoxide, acetyl chloride/dimethyl sulfoxide, acetyl anhydride/dimethyl sulfoxide, trifluoroacetyl chloride/dimethyl sulfoxide, toluenesulfonyl bromide/dimethyl sulfoxide, phosphorous pentachloride/dimethyl sulfoxide and isobutylchloroformate/dimethyl sulfoxide.
A general Scheme for the preparation of amino epoxides, useful as intermediates in the synthesis of HIV protease inhibitors is shown in Scheme 1 below. 
The economical and safe large scale method of preparation of protease inhibitors of the present invention can alternatively utilize amino acids or amino alcohols to form N,N-protected alpha aminoalcohol of the formula 
wherein P1, P2 and R1 are described above.
Whether the compounds of Formula II are formed from amino acids or aminoalcohols, such compounds have the amine protected with groups P1 and P2 as previously identified. The nitrogen atom can be alkylated such as by the addition of suitable alkylating agents in an appropriate solvent in the presence of base.
Alternate bases used in alkylation include sodium hydroxide, sodium bicarbonate, potassium hydroxide, lithium hydroxide, potassium carbonate, sodium carbonate, cesium hydroxide, magnesium hydroxide, calcium hydroxide or calcium oxide, or tertiary amine bases such as triethyl amine, diisopropylethylamine, N-methylpiperidine, pyridine, dimethylaminopyridine and azabicyclononane. Reactions can be homogenous or heterogenous. Suitable solvents are water and protic solvents or solvents miscible with water, such as methanol, ethanol, isopropyl alcohol, tetrahydrofuran and the like, with or without added water. Dipolar aprotic solvents may also be used with or without added protic solvents including water. Examples of dipolar aprotic solvents include acetonitrile, dimethylformamide, dimethyl acetamide, acetamide, tetramethyl urea and its cyclic analog, dimethylsulfoxide, N-methylpyrrolidone, sulfolane, nitromethane and the like. Reaction temperature can range between about xe2x88x9220xc2x0 to 100xc2x0 C. with the preferred temperature of about 25-85xc2x0 C. The reaction may be carried out under an inert atmosphere such as nitrogen or argon, or normal or dry air, under atmospheric pressure or in a sealed reaction vessel under positive pressure. The most preferred alkylating agents are benzyl bromide or benzyl chloride or monosubstituted aralkyl halides or polysubstituted aralkyl halides. Sulfate or sulfonate esters are also suitable reagents to provide the corresponding benzyl analogs and they can be preformed from the corresponding benzyl alcohol or formed in situ by methods well known to those skilled in the art. Trityl, benzhydryl, substituted trityl and substituted benzhydryl groups, independently, are also effective amine protecting groups [P1,P2] as are allyl and substituted allyl groups. Their halide derivatives can also be prepared from the corresponding alcohols by methods well known to those skilled in the art such as treatment with thionyl chloride or bromide or with phosphorus tri- or pentachloride, bromide or iodide or the corresponding phosphoryl trihalide. Examples of groups that can be substituted on the aryl ring include alkyl, alkoxy, hydroxy, nitro, halo and alkylene, amino, mono- and dialkyl amino and acyl amino, acyl and water solubilizing groups such as phosphonium salts and ammonium salts. The aryl ring can be derived from, for example, benzene, napthelene, indane, anthracene, 9-(9-phenyl fluorenyl, durene, phenanthrene and the like. In addition, 1,2-bis (substituted alkylene) aryl halides or sulfonate esters can be used to form a nitrogen containing aryl or non-aromatic heterocyclic derivative [with P1 and P2] or bis-heterocycles. Cycloalkylenealkyl or substituted cyloalkylene radicals containing 6-10 carbon atoms and alkylene radicals constitute additional acceptable class of substituents on nitrogen prepared as outlined above including, for example, cyclohexylenemethylene.
Compounds of Formula II can also be prepared by reductive alkylation by, for example, compounds and intermediates formed from the addition of an aldehyde with the amine and a reducing agent, reduction of a Schiff Base, carbinolamine or enamine or reduction of an acylated amine derivative. Reducing agents include metals [platinum, palladium, palladium hydroxide, palladium on carbon, platinum oxide, rhodium and the like] with hydrogen gas or hydrogen transfer molecules such as cyclohexene or cyclohexadiene or hydride agents such as lithium aluminumhydride, sodium borohydride, lithium borohydride, sodium cyanoborohydride, diisobutylaluminum hydride or lithium tri-tert-butoxyaluminum hydride.
Additives such as sodium or potassium bromide, sodium or potassium iodide can catalyze or accelerate the rate of amine alkylation, especially when benzyl chloride was us ed as the nitrogen alkylating agent.
Phase transfer catalysis wherein the amine to be protected and the nitrogen alkylating agent are reacted with base in a solvent mixture in the presence of a phase transfer reagent, catalyst or promoter. The mixture can consist of, for example, toluene, benzene, ethylene dichloride, cyclohexane, methylene chloride or the like with water or a aqueous solution of an organic water miscible solvent such as THF. Examples of phase transfer catalysts or reagents include tetrabutylammonium chloride or iodide or bromide, tetrabutylammonium hydroxide, tri-butyloctylammonium chloride, dodecyltrihexylammonium hydroxide, methyltrihexylammonium chloride and the like.
A preferred method of forming substituted amines involves the aqueous addition of about 3 moles of organic halide to the amino acid or about 2 moles to the aminoalcohol. In a more preferred method of forming a protected amino alcohol, about 2 moles of benzylhalide in a basic aqueous solution is utilized. In an even more preferred method, the alkylation occurs at 50xc2x0 C. to 80xc2x0 C. with potassium carbonate in water, ethanol/water or denatured ethanol/water. In a more preferred method of forming a protected amino acid ester, about 3 moles of benzylhalide is added to a solution containing the amino acid.
The protected amino acid ester is additionally reduced to the protected amino alcohol in an organic solvent. Preferred reducing agents include lithium aluminumhydride, lithium borohydride, sodium borohydride, borane, lithium tri-ter-butoxyaluminum hydride, borane-THF complex. Most preferably, the reducing agent is diisobutylaluminum hydride (DiBAL-H) in toluene. These reduction conditions provide an alternative to a lithium aluminum hydride reduction.
Purification by chromatography is possible. In the preferred purification method the alpha amino alcohol can be purified by an acid quench of the reaction, such as with hydrochloric acid, and the resulting salt can be filtered off as a solid and the amino alcohol can be liberated such as by acid/base extraction.
The protected alpha amino alcohol is oxidized to form a chiral amino aldehyde of the formula 
Acceptable oxidizing reagents include, for example, sulfur trioxide-pyridine complex and DMSO, oxalyl chloride and DMSO, acetyl chloride or anhydride and DMSO, trifluoroacetyl chloride or anhydride and DMSO, methanesulfonyl chloride and DMSO or tetrahydrothiaphene-S-oxide, toluenesulfonyl bromide and DMSO, trifluoromethanesulfonyl anhydride (triflic anhydride) and DMSO, phosphorus pentachloride and DMSO, dim ethylphosphoryl chloride and DMSO and isobutylchlordformate and DMSO. The oxidation conditions reported by Reetz et al [Agnew Chem., 99, p. 1186, (1987)], Agnew Chem. Int. Ed. Engl., 26, p. 1141, 1987) employed oxalyl chloride and DMSO at xe2x88x9278xc2x0 C.
The preferred oxidation method described in this invention is sulfur trioxide pyridine complex, triethylamine and DMSO at room temperature. This system provides excellent yields of the desired chiral protected amino aldehyde usable without the need for purification i.e., the need to purify kilograms of intermediates by chromatography is eliminated and large scale operations are made less hazardous. Reaction at room temperature also eliminated the need for the use of low temperature reactor which makes the process more suitable for commercial production.
The reaction may be carried out under an inert atmosphere such as nitrogen or argon, or normal or dry air, under atmospheric pressure or in a sealed reaction vessel under positive pressure. Preferred is a nitrogen atmosphere. Alternative amine bases include, for example, tri-butyl amine, tri-isopropyl amine, N-methylpiperidine, N-methyl morpholine, azabicyclononane, diisopropylethylamine, 2,2,6,6-tetramethylpiperidine, N,N-dimethylaminopyridine, or mixtures of these bases. Triethylamine is a preferred base. Alternatives to pure DMSO as solvent include mixtures of DMSO with non-protic or halogenated solvents such as tetrahydrofuran, ethyl acetate, toluene, xylene, dichloromethane, ethylene dichloride and the like. Dipolar aprotic co-solvents include acetonitrile, dimethylformamide, dimethylacetamide, acetamide, tetramethyl urea and its cyclic analog, N-methylpyrrolidone, sulfolane and the like. Rather than N,N-dibenzylphenylalaninol as the aldehyde precursor, the phenylalaninol derivatives discussed above can be used to provide the corresponding N-monosubstituted [either P1 or P2=H] or N,N-disubstituted aldehyde.
In addition, hydride reduction of an amide or ester derivative of the corresponding alkyl, benzyl or cycloalkenyl nitrogen protected phenylalanine, substituted phenylalanine or cycloalkyl analog of phenyalanine derivative can be carried out to provide a compound of Formula III. Hydride transfer is an additional method of aldehyde synthesis under conditions where aldehyde condensations are avoided, cf, Oppenauer Oxidation.
The aldehydes of this process can also be prepared by methods of reducing protected phenylalanine and phenylalanine analogs or their amide or ester derivatives by, e.g., sodium amalgam with HCl in ethanol or lithium or sodium or potassium or calcium in ammonia. The reaction temperature may be from about xe2x88x9220xc2x0 C. to about 45xc2x0 C., and preferably from abut 5xc2x0 C. to about 25xc2x0 C. Two additional methods of obtaining the nitrogen protected aldehyde include oxidation of the corresponding alcohol with bleach in the presence of a catalytic amount of 2,2,6,6-tetramethyl-1-pyridyloxy free radical. In a second method, oxidation of the alcohol to the aldehyde is accomplished by a catalytic amount of tetrapropylammonium perruthenate in the presence of N-methylmorpholine-N-oxide.
Alternatively, an acid chloride derivative of a protected phenylalanine or phenylalanine derivative as disclosed above can be reduced with hydrogen and a catalyst such as Pd on barium carbonate or barium sulphate, with or without an additional catalyst moderating agent such as sulfur or a thiol (Rosenmund Reduction).
An important aspect of the present invention is a reaction involving the addition of chloromethylithium or bromomethyllithium to the xcex1-amino aldehyde. Although addition of chloromethyllithium or bromomethylithium to aldehydes is known, the addition of such species to racemic or chiral amino aldehydes to form aminoepoxides of the formula 
is novel. The addition of chloromethylithium or bromomethylithium to a chiral amino aldehyde is highly diastereoselective. Preferably, the chloromethyllithium or bromomethylithium is generated in-situ from the reaction of the dihalomethane and n-butyllithium. Acceptable methyleneating halomethanes include chloroiodomethane, bromochloromethane, dibromomethane, diiodomethane, bromofluoromethane and the like. The sulfonate ester of the addition product of, for example, hydrogen bromide to formaldehyde is also a methyleneating agent. Tetrahydrofuran is the preferred solvent, however alternative solvents such as toluene, dimethoxyethane, ethylene dichloride, methylene chloride can be used as pure solvents or as a mixture. Dipolar aprotic solvents such as acetonitrile, DMF, N-methylpyrrolidone are useful as solvents or as part of a solvent mixture. The reaction can be carried out under an inert atmosphere such as nitrogen or argon. For n-butyl lithium can be substituted other organometalic reagents reagents such as methyllithium, tert-butyl lithium, sec-butyl lithium, phenyllithium., phenyl sodium and the like. The reaction can be carried out at temperatures of between about xe2x88x9280xc2x0 C. to 0xc2x0 C. but preferably between about xe2x88x9280xc2x0 C. to xe2x88x9220xc2x0 C. The most preferred reaction temperatures are between xe2x88x9240xc2x0 C. to xe2x88x9215xc2x0 C. Reagents can be added singly but multiple additions are preferred in certain conditions. The preferred pressure of the reaction is atmospheric however a positive pressure is valuable under certain conditions such as a high humidity environment.
Alternative methods of conversion to the epoxides of this invention include substitution of other charged methylenation precurser species followed by their treatment with base to form the analogous anion. Examples of these species include trimethylsulfoxonium tosylate or triflate, tetramethylammonium halide, methyldiphenylsulfoxonium halide wherein halide is chloride, bromide or iodide.
The conversion of the aldehydes of this invention into their epoxide derivative can also be carried out in:multiple steps. For example, the addition of the anion of thioanisole prepared from, for example, a butyl or aryl lithium reagent, to the protected aminoaldehyde, oxidation of the resulting protected aminosulfide alcohol with well known oxidizing agents such as hydrogen peroxide, tert-butyl hypochlorite, bleach or sodium periodate to give a sulfoxide. Alkylation of the sulfoxide with, for example, methyl iodide or bromide, methyl tosylate, methyl mesylate, methyl triflate, ethyl bromide, isopropyl bromide, benzyl chloride or the like, in the presence of an organic or inorganic base. Alternatively, the protected aminosulfide alcohol can be alkylated with, for example, the alkylating agents above, to provide sulfonium salts that are subsequently converted into the subject epoxides with tert-amine or mineral bases.
The desired epoxides form, using most preferred conditions, diastereoselectively in ratio amounts of at least about an 85:15 ratio (S:R). The product can be purified by chromatography to give the diastereomerically and enantiomerically pure product but it is more conveniently used directly without purification to prepare HIV protease inhibitors.
This process is applicable to mixtures of optical isomers as well as resolved compounds. If a particular optical isomer is desired, it can be selected by the choice of starting material, e.g., L-phenylalanine, D-phenylalanine, L-phenylalaninol, D-phenylalaninol, D-hexahydrophenylalaninol and the like, or resolution can occur at intermediate or final steps. Chiral auxiliaries such as one or two equivalents of camphor sulfonic acid, citric acid, camphoric acid, 2-methoxyphenylacetic acid and the like can be used to form salts, esters or amides of the compounds of this invention. These compounds or derivatives can be crystallized or separated chromatographically using either a chiral or a chiral column as is well known to those skilled in the art.
A further advantage of the present process is that materials can be carried through the above steps without purification of the intermediate products. However, if purification is desired, the intermediates disclosed can be prepared and stored in a pure state.
The practical and efficient synthesis described here has been successfully scaled up to prepare large quantity of intermediates for the preparation of HIV protease inhibitors. It offers several advantages for multikilogram preparations: (1) it does not require the use of hazardous reagents such as diazomethane, (2) it requires no purification by chromatography, (3) it is short and efficient, (4) it utilizes inexpensive and readily available commercial reagents, (5) it produces enantiomerically pure alpha amino epoxides. In particular, the process of the invention produces enantiomerically-pure epoxide as required for the preparation of enantiomerically-pure intermediate for further synthesis of HIV protease inhibitors.
The amino epoxides were prepared utilizing the following procedure as disclosed in Scheme II below. 
In Scheme II, there is shown a synthesis for the epoxide, chiral N,N,xcex1-S-tris(phenylmethyl)-2S-oxiranemethan-amine. The synthesis starts from L-phenylalanine. The aldehyde is prepared in three steps from L-phenylalanine or phenylalinol. L-Phenylalanine is converted to the N,N-dibenzylamino acid benzyl ester using benzyl bromide under aqueous conditions. The reduction of benzyl ester is carried out using diisobutylaluminum hydride (DIBAL-H) in toluene. Instead of purification by chromatography, the product is purified by an acid (hydrochloric acid) quench of the reaction, the hydrochloride salt is filtered off as a white solid and then liberated by an acid/base extraction. After one recrystallization, chemically and optically pure alcohol is obtained. Alternately, and preferably, the alcohol can be obtained in one step in 88% yield by the benzylation of L-phenylalaninol using benzylbromide under aqueous conditions. The oxidation of alcohol to aldehyde is also modified to allow for more convenient operation during scaleup. Instead of the standard Swern procedures using oxalyl chloride and DMSO in methylene chloride at low temperatures (very exothermic reaction), sulfur trioxide-pyridine/DMSO was employed (Parikh, J., Doering, W., J. Am. Chem. Soc., 89, p. 5505, 1967) which can be conveniently performed at room temperature to give excellent yields of the desired aldehyde with high chemical and enantiomer purity which does not require purification.
An important reaction involves the addition of chloromethylithium or bromomethylithium to the aldehyde. Although addition of chloromethyllithium or bromomethylithium to aldehydes has been reported previously, the addition of such species to chiral xcex1-amino aldehydes to form chiral-aminoepoxides is believed to be novel. Now, chloromethyllithium or bromomethylithium is generated in-situ from chloroiodomethane(or bromochloromethane) or dibromomethane and n-butyllithium at a temperature in a range from about xe2x88x9278xc2x0 C. to about xe2x88x9210xc2x0 C. in THF in the presence of aldehyde. The desired chlorohydrin or bromohydrin is formed as evidenced by TLC analyses. After warming to room temperature, the desired epoxide is formed diastereoselectively in a 85:15 ratio (S:R). The product can be purified by chromatography to give the diastereomerically pure product as a colorless oil but it is more conveniently used directly without purification.